Digital subscriber line (xDSL) technology has developed in recent years in response to the demand for high-speed Internet access. xDSL technology utilizes the communication medium of pre-existing telephone systems. Thus, both plain old telephone systems (POTS) and xDSL systems share a common line for xDSL-compatible customer premises. Similarly, other services such as time compression multiplexing (TCM) integrated services digital network (ISDN) can also share a common line with xDSL and POTS.
Allocations of wire pairs within telephone cables in accordance with service requests have typically resulted in a random distribution of pair utilization with few precise records of actual configurations. Because of the physical proximity of bundled cables (due to pair twisting, cable branching, cable splicing, etc.), crosstalk caused by the electromagnetic interference between the neighboring lines is often the dominating noise source in the transmission environment. In addition, due to pair twisting in cables where cable branching and splicing take place, a wire pair can be in close proximity to many different pairs spanning different portions of its length. At a telephone CO (central office), pairs in close proximity may carry diverse types of service using various modulation schemes, with considerable differences in signal levels (and receiver sensitivities) especially for pairs of considerably different lengths.
There are generally two types of crosstalk mechanisms that are characterized, one being FEXT and the other one being near-end crosstalk (NEXT). FEXT refers to electromagnetic coupling that occurs when the receiver on a disturbed pair is located at the far end of the communication line as the transmitter of a disturbing pair. Self induced far end crosstalk (self-FEXT) generally refers to interference caused by neighboring lines provisioned for the same type of service as the affected line, or “victim line.” In contrast, NEXT results from a disturbing source connected at one end of the wire pair which causes interference in the message channel at the same end as the disturbing source. Current approaches to addressing crosstalk suffer from various perceived shortcomings. While vectored systems are effective in addressing such disturbances as self-FEXT, various types of interferences such as radio frequency interference (RFI), power line communications (PLC), common mode noise, and impulse nose remain an issue.